The present invention relates to magnetic resonance imaging (MRI) method and apparatus, particularly to techniques for removing artifacts in ultra fast MR imaging methods.
MRI devices measure spatial distributions of proton density within an object to be examined and spatial distributions of relaxation times from excited states, and form images from measured data. In this manner, MRI devices reveal anatomical and physiological information regarding the human body, head, retroperitoneum, or the like, and create two-dimensional or three-dimensional images for display.
MRI imaging involves applying phase encoded gradient magnetic fields to nuclear spins excited by radio frequency magnetic fields to generate echo signals. The phase encode levels usually are 128, 256, 512 or the like per image. Each echo signal usually is sampled at 128, 256, 512, or 1,024 sampling points, during the application of readout gradient magnetic fields. Two-dimensional Fourier transformation is performed on the measured data to create or reconstruct MR images of, e.g., 256xc3x97256 pixels each.
An EPI (Echo Planar Imaging) method has been used as a high-speed imaging method for MRI. In image data derived by the EPI method, a phase errors can be introduced that causes artifacts. The main sources of such phase error in image data from the EPI method include non-uniformities in the MRI static magnetic field and eddy currents caused by the inversion of gradient magnetic field.
An ETS (echo time shifting) technique has been developed to reduce artifacts caused by phase error due to non-uniformities in the static magnetic field. The ETS method is described in publications such as xe2x80x9cPhase Error Corrected Interlaced Echo Planar Imagingxe2x80x9d; Z. H Cho and C. B Ahn et. al.; Proceedings of Annual Meetings of the Society of Magnetic Resonance in Medicine (=SMRM), No.912, 1987, and xe2x80x9cPhase Error in Multi-Shot Echo Planar Imagingxe2x80x9d; David A. Feinberg and Koichi Oshio: Magnetic Resonance in Medicine, Vol.32, 535-539 (1994), etc. These publications pertain to configuring pulse sequences in data acquisition. Techniques also have been proposed for reducing artifacts due to both non-uniformities in the static magnetic field and eddy currents generated by inversion of polarity of gradient magnetic fields. Such techniques seek to compensate phase values of image data by reference data being acquired to detect turbulence of phase of echo signals. See, for example, Japanese Patent Laid-Open No. Heisei 5-68674.
Conventionally, in the case of MR imaging performed by an EPI method pulse sequence, the ETS method or the phase correction method are used independently. Each method can reduce artifacts significantly but not remove them completely. If the two methods are combined, it could be thought that the beneficial effect would increase, but corrections performed on data acquired by the ETS method through using reference data may not actually improve image quality, rather it can make an artifact be seen more clearly.
The inventors herein have found a solution that involves acquiring the data in an ETS method at different data acquisition timing for each shot, thereby making the phase of signals different for different shots.
Even if correction is performed by a single set of reference data, the correction may not be performed precisely; accordingly image quality may suffer.
The object of the present invention is to improve image quality of MR images acquired by an EPI method.
Another object of the present invention is to provide magnetic resonance imaging method and MRI apparatus capable of acquiring a good MR image by combining an ETS method and a phase correction method.
In order to solve such problems in this invention, an ETS method and a phase correction method using a plurality of sets of reference data corresponding to a plurality of shots, are combined and performed.
In addition, although the overall imaging time including acquisition of both an image data set and a reference data set may be longer theoretically than when acquiring only an image data set, this invention also discloses an improvement directed to this problem. That is, in order to improve time efficiency by shortening the overall imaging time, reference data is acquired in only one shot, time development of phase error is estimated from the reference data acquired in said one shot, the phase of reference data to all shots is estimated, and phase correction for the entire image data set is thus enabled.
In one embodiment, the present invention comprises a magnetic resonance imaging apparatus having an RF excitation pulse generator, static magnetic field generating means, gradient magnetic field generating coils, high frequency echo detecting coil, and control means for controlling the acquisition of image data from a series of RF echo signals. The echo signals are acquired in respective shots in repetition time periods TR. In each shot, a repetition time TR includes an RF excitation pulse and repeated application of an alternating polarity readout gradient magnetic field. The control means controls the time interval between application of the excitation pulse and the start of the repeated inversions of polarity. Image data acquired with the application of phase encoding gradient magnetic fields is phase corrected by using reference data acquired without phase encoding gradient magnetic fields, or at least substantially without such fields.
The phase correction can involve correcting Fourier transformed image data by using phase information from Fourier transformed reference data, and can involve one-dimensional Fourier transformation of each of said reference data and said image data in the readout direction.
Phase correction can be performed by subtracting phase information of Fourier transformed reference data from phase information of Fourier transformed image data.
The reference data can be derived from actual measurement for a certain time interval; for other time intervals can it be derived from calculations based on the actually measured reference data.
Phase information can be obtained from the transformed reference data resulting from a one-dimensional Fourier transformation of the reference data measured in a readout direction.
Phase change information can also be obtained by calculating phase changes corresponding to difference of time intervals between application of RF excitation pulses and the start of application of gradient magnetic fields with inversion of polarity by using time development of phase distortion estimated from phase information of said transformed reference data.
Time development of phase distortion can be estimated by linear approximation or interpolation.
Estimates of time development of phase distortion can be derived by inter polation or extra polation from phase differences between echoes acquired over a period of time.
Another embodiment of the invention comprises a magnetic resonance imaging method including the steps of (1) applying an RF excitation pulse, and (2) acquiring data by detecting a series of echo signals for readout gradient magnetic field pulses of alternating polarity. The time interval between step (1) and the start of step (2) can vary for different shots. Phase correction can be applied to image data acquired with phase encoding gradient magnetic fields by using reference data obtained without use of substantial (or any) encoding gradient magnetic fields.
Image quality is improved by using an ETS method in combination with a phase correction method, using reference data corresponding to plural shots. Such improvement can exceed the improvement resulting from using either method independently. In addition, imaging time can be shortened by acquiring reference data with only one shot (or few shots) and estimating other reference data, as compared to the time taken for actually measuring reference data for all or a greater number of shots corresponding to the shots for image data.